Thermoanemometer-type thermal sensors have been commonly used to measure flow and differential pressure in gases and liquids. Typically the sensing element of the sensor contains a central heater and two temperature-sensitive entities positioned symmetrically on both sides of the heater. Gas or liquid (fluid) flow passing over the sensing element disturbs the temperature distribution in the fluid in the vicinity of the heater, causing a temperature difference between two temperature-sensitive entities, which is further converted into an electrical signal.
Some packaging solutions for flow sensors include a specially-designed flow channel within which the sensing element is positioned, such that the fluid flow passes over the sensing element or surrounds the sensing element as it passes. Typically the flow channel maintains laminar flow which is directly proportional to the pressure difference between the two ports (ends) of the flow channel. Therefore these devices may be considered as differential pressure sensors and/or flow sensors.
One prior art sensor design comprises a package cover with two ports and a groove in its surface. After this cover is attached to the base of the package, with the sensor chip mounted to the base, the flow channel is formed by the groove together with the surface of the sensor chip. With the sensor chip mounted to the base of the package, the flow channel is defined by this groove together with the surface of the sensor chip. The sensor chip is aligned such that the fluid in the channel flows over or surrounding the sensing element, causing the output signal.
Another sensor design comprises the cover of the package with two pre-molded cavities to form the gas flow channel and provide means to cover/protect bonding wires.
A common feature of these above two sensor packaging schemes, and other analogous sensor packaging schemes, is the formation of the flow channel at the stage of package assembly, by mating the individual sensor chip with package components. Typically, an individual flow channel is mated to the sensor chip after the sensor chip has been mounted on or in the package base. Variations in the sensor performance may come from possible misalignment of the sensor chip to the package components, and from unit-to-unit variability of the dimensions of the flow channel, both sources influenced by the manufacturing process of the packaging components and by the unit-by-unit sensor-assembly operation.
Therefore, there is a need to reduce the impact of the packaging process on the creation of flow sensor devices to minimize variations in performance of individual devices.